<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <div id="app"></div>
  <template id="my-app">
    <h2>{{count}}</h2>
    <button @click="handleAdd($event, 10)">add</button>
  </template>

  <script src="https://cdn.bootcdn.net/ajax/libs/vue/3.3.4/vue.global.js"></script>
  <script>
    Vue.createApp({
      template: '#my-app',
      data() {
        return {
          count: 1
        }
      },
      methods: {
        handleAdd(e, n) {
          console.log(e);
          this.count += n
        }
      }
    }).mount('#app')
  </script>
</body>
</html>